Support for drag 'n drop

[l0drex]

It would be nice if the plugin would support drag 'n drop of heic images into the wallpaper view.

This would make the process of adding an image more comfortable: If the image is already available in another application like Dolphin, since the user wouldn't need to click the Add button and search for the image again.

For more information on how to do drag 'n drop in Qt, see the documentation here.